In a die bonding operation in a semiconductor assembly and manufacturing process, a bonding tool may pick up a die (which may be an electronic device such as a semiconductor integrated circuit device or semiconductor chip) at a first location and move the die to a second location at which the die is to be bonded. Commonly, the die is bonded to a substrate, such as a circuit board, or another die. At the bonding location, the bonding tool will move downwardly (this is known as the z-direction) until the die touches the substrate or another die. In order to apply a required bond force during the bond operation, the bonding tool may be driven downwardly further to exert the required force on the die.
Apparatus and methods for controlling this bond force need to meet a number of requirements. For example, they should preferably be able to apply a bond force over a wide range, should be susceptible to feedback and control, and should be able to respond rapidly to required changes in the bond force.
There are demanding requirements placed on die tilt and bond line thickness specifications. The bonding tool usually has a collet that is used to contact a die to be bonded. The collet is often made of metals like stainless steel or tungsten carbide, with a compliant material (such as rubber material) added to the tip of the collet to form a contact surface to hold the die. The collet includes a suction aperture through which a holding force can be provided by air suction. The contacting surface of the collet of the bonding tool should be parallel to a pick-up surface and a placement surface so that a bonding force is evenly distributed onto a die to be picked and placed, and the die can be placed correctly onto a bonding position.
If a bonding tool is not aligned properly, so that the contacting surface is not substantially parallel to the placement surface, die crack or an unacceptable placement error may occur. In order to produce good results, the bonding tool of a modern die bonder needs to be carefully set up to achieve leveling that is better than 16 microns. In other words, a vertical distance between a lowest point on a die and a highest point on the die when carried by the bonding tool must be smaller than 16 microns.
One method of aligning the bonding tool is to mount a circular stamp on the bonding tool and then to land the stamp on a perfectly flat anvil block. A carbon paper is placed on the anvil block to obtain an imprint of the circular stamp. By checking the roundness or completeness of the imprint, an operator is able to visually determine whether the bonding tool has been set up correctly to achieve an acceptable leveling. If the imprint shows an incomplete circle, the operator may correct the alignment of the bonding tool according to the tilting direction of the bonding tool as interpreted from the imprint. This method is manual and not very accurate, since it relies on subjective visual determination by the operator.
Another method of aligning a bonding tool is disclosed in U.S. Pat. No. 6,179,938 for “Method and Apparatus for Aligning the Bonding Head of a Bonder, In Particular a Die Bonder”. In this patent, an alignment plate provided with two plane parallel surfaces is placed on a supporting surface which is set plane parallel to the bonding surface upon which the semiconductor chip will be bonded to a carrier material. A measuring device is then calibrated, after which the alignment plate is held at a slight distance above the measuring device. The alignment of the bonding tool is adjusted until the signal from the measuring device is equal to the signal obtained during calibration.
A disadvantage of this method is that there has to be prior calibration each time the bonding tool is to be aligned. The inductive range-finding method used by the alignment apparatus requires prior calibration each time alignment is to be measured, since it uses separate coils that are susceptible in different ways to external influences such as temperature, humidity and so forth. Each coil also forms a magnetic field that may influence the other coils, thus affecting accuracy. Furthermore, the bonding tool has to pick up a specially-prepared alignment plate and move it over the measuring device each time alignment is to be determined. Evidently, closed-loop control to maintain alignment of the bonding tool during bonding will not be possible.